It's not 100% Rust but I think you would expect that GTK itself is fairly well tested - probably much better tested than any new app you write that uses it, so it is still worth it to write that app in Rust.

And it has to be said that memory safety isn't Rust's only compelling feature. It also has a pretty great build system, a decent library ecosystem, a very strong type system which gives you an "if it compiles it works" experience surprisingly often, probably the best multithreading system, etc. etc.